This invention concerns a key switch which could, for example, be used to switch a power supply for a machine tool on and off. In general, a key switch is generally defined here as an electrical switch device which has a key to allow the access to the switch.
It would be extremely dangerous for a machine tool to be switched on accidentally and operated unexpectedly. For this reason, the area around the tool is usually surrounded by a cage. When the machine tool is to be repaired or inspected, it can be approached through a door in the cage which has been furnished for that purpose. This door has a safety switch to prevent the machine tool from operating when the actuator, which is outside the cage, is accidentally activated.
This safety switch can be the switch for the machine tool's power supply. For example, a key can be hung by a short string on the door and a switch positioned in such a way that the key fits into the switch only when the door is closed. The power supply can then be switched on only when the door is closed and the key inserted.
However, with this prior-art type of safety switch, it is possible for the machine tool to be activated even though the key has not been inserted, if it should happen that some other object be accidentally inserted. Thus this switch is not failsafe. It might happen, for example, that vibration causes something to find its way into the switch when the door has not been closed and the key is not in the switch. The machine tool will then be activated unintentionally even though the door is open, and an accident may occur. This invention was designed with this scenario in mind.
The first object of this invention is to provide a key switch with better operating safety in light of the above problem.
The second object of this invention is to provide a further safety key switch which does not allow a key to open the door until the switch detects that the machine is completely stopped, in light of the following problem.
In an actual operation, there is still a possibility that inertia may cause the machine tool to continue to rotate after the door is opened, even though it is prevented from operating under power at that time. Safety cannot be guaranteed in such a case.
To produce a safety switch which can address this problem, a conventional key switch uses an electromagnetic solenoid and an auxiliary switch. The electromagnetic solenoid locks the key in place so that it cannot be removed freely from the switch, and the auxiliary switch is actuated by this solenoid. This auxiliary switch is connected in series with the internal main switch.
With this conventional safety switch, the electromagnetic solenoid is driven by a signal conveying the information that the rotation of the machine tool has ceased. This releases the lock on the key. Thus it is only when the machine tool has absolutely stopped operating that it becomes possible to extract the key to open the door. However, there is a likelihood that a rotation detector will make an error and a "rotation stopped" signal will be output even though the machine tool was actually rotating. The key could then be removed when the key lock was released, and the door could be opened. At this time the machine tool might still be rotating due to the effects of inertia. Since the auxiliary switch is switched off by the electromagnetic solenoid when the lock is released, this arrangement allows the machine tool to be stopped as soon as possible after the key is removed.
However, with the prior art key switch of the type described above, the electromagnetic solenoid and the auxiliary switch are a single unit, and the internal switch actuated by the key is connected in series to the auxiliary switch. This arrangement requires sufficient space to install both switches as well as space for the wiring between them, so the resulting switch is relatively large.
Furthermore, the use of two separate switches drives up the cost of the key switch. Production of such a switch requires a large number of processes, including the installation of the two switches and the laying of the wiring between them. These difficulties pose obstacles to the improvement of efficiency of assembly and productivity.
In view of these problems, the objective of this invention is to provide a key switch which features a solenoid to lock the key, and which uses only one internal switch. This enables the key switch to be downsized, and reduces the number of production processes. This in turn can lower production cost and improve assembly efficiency and productivity.